INFLUENCE OF RING SUBSTITUENTS AND MATRIX ON LITHIUM SODIUM SELECTIVITY OF 14-CROWN-4 AND BENZO-13-CROWN-4 COMPOUNDS

Binding of Li+ and Na+ by a series of ten benzo-13-crown-4 compounds and a series of fifteen 14-crown-4 derivatives has been assessed by picrate extraction and the response of polymeric membrane electrodes. Variation in cation selectivity was observed when different groups were attached to the central carbon atom of the three-carbon bridges in the ionophores. For the benzo-13-crown-4 series, only weak cation binding and low selectivity was observed in both the picrate extraction and polymeric membrane electrode systems. For the 14-crown-4 series, 6,13-dimethylenyl-14-crown-4 exhibited the highest Li+ selectivity in picrate extraction. In polymeric membrane electrodes, considerably higher Li+ selectivity was observed with 6,6,13-tribenzyl-14-crown-4 and 6,6,13,13-tetraethyl-14-crown-4 than with the commercially available Li+ ionophore, 6,6-dibenzyl-14-crown-4. Lack of correlation for the Li+ selectivities of the 14-crown-4 compounds in the picrate extraction and polymeric membrane electrode systems reveals a profound influence of the matrix on ionophore selectivity.